First, find your SteamID. You can find your SteamID in one of three ways:

1) —

In-game, press ~ (tilde) to open the Console, and enter the command status . Locate your name, and next to it you will see your SteamID. For example, [U:1:XXXXXX]. If your Console is disabled, open the Options window on the TF2 Main Window. In the Keyboard tab, click the Advanced button, and check "Enable Developer Console".